High temperature (HT) heat treatment of semiconductor wafers is generally be carried out with significant precautions in order to avoid damaging the wafer's structure (or at least of some of its layers).
Wafers are usually disposed into a receiving structure called a “boat” in order to carry out the heat treatment and specific boats have been developed for minimizing specific type of damage, e.g., slip line generation, during HT heat treatments. FIG. 1 shows an exemplary “low slip line boat” boat 10 made of SiC. This boat comprises fingers 11 and 12 for maintaining each wafer in a horizontal position in the heating chamber during HT heat treatment. For this purpose, the fingers are in contact with the back surface of the wafer.
Although FIG. 1 is a side view showing only one finger 11 and one finger 12, such boats typically comprises several sets of fingers 11 and 12 defining several slots for receiving multiple wafers. Each slot is defined by two adjacent sets of fingers and supports one wafer lying normally in the horizontal plane.
In many types of such boats hold, the fingers hold and support wafers in regions near the wafer center in order to decrease constraints applied to the wafer and thus also decrease the number of the associated slip lines. Other types of boats have fingers which support the wafer only in its edge region. Such configurations tend to create increased mechanical constraints and increased numbers of slip lines.
However, generally low slip line boats, e.g., the boat illustrated in FIG. 1, reduce the number of slip lines generated during HT heat treatments. This reduction is believed to derive from the large surface of contact between the fingers of the boat and the back surface of the wafer.
It has been observed that when SOI wafers are subject to HT heat treatments in such boats, the wafers tend to have defects associated to tearing-off (“tearing-off defects”) of at least a portion of one layer. Such tearing-off defects also occur for other type of multilayer wafers. Tearing-off defects correspond to a tearing-off of at least a portion of the surface layer of the wafer and are therefore clearly different from defects in the crystallographic structure of the wafer material (such as slip lines or dislocations). The present invention now provides a solution to this problem.